Numerous types of litter boxes have been proposed. There are two predominant types; one provides a simple filtering mechanism, and the other, a fairly complex dump-and-replace mechanism. Filtering mechanisms normally work by sifting dirty litter material through a filter-like screen in order to separate hard waste refuse from the litter itself. Menzel (U.S. Pat. No. 4,190,525) is an example of such mechanisms. This approach is normally inadequate, as odor problems are due to ammonia-filled liquid waste rather than to hard waste. Removing hard waste elements from the litter simply does not solve the problem of the litter box, and forces users into a cumbersome two-step cleaning process in which hard waste is cleaned first and the litter material itself is replaced at a later time. Such systems require double work.
The dump-and-replace types clean hard and liquid waste at the same time by dumping all soiled litter into a trash receptacle. These systems usually include a "litter reservoir" that stores clean litter, an openable/closable trap-like bottom tray that holds the litter to be used by a pet, and a linable receptacle located beneath the tray to receive and hold used litter. Maness (U.S. Pat. No. 3,954,086) and Van der Kolk (U.S. Pat. No. 4,493,288) provides examples of such systems. Existing designs of the dump-and-replace approach contend with two important problems that impair their successful commercialization. They require sophisticated electronic sensors to coordinate the opening/closing of the litter reservoir with the closing/opening of the bottom tray. They also need an electrical mechanism to deal with the significant weight of the litter to be lifted and displaced. The high cost of electronics and electrical components and the potential danger accompanying the use of such components render the resulting mechanisms unfit for practical implementation.
Further examples of the development of litter boxes are provided by Printz (U.S. Pat. No. 3,621,817), Taylor (U.S. Pat. No. 3,908,597), Cotter (U.S. Pat. No. 4,096,827), Bilak (U.S. Pat. No. 4,327,667) and Mopper (U.S. Pat. No. 4,465,018).